Electronic Design

Happy Landings—And Lots Of Them

It’s a familiar scenario. You’re flying home from a busy business trip or hectic holiday. You’re tired and just want to get off the aircraft and wend your way home. Then the pilot announces that because of poor visibility affecting airport flight paths, the aircraft is going into a holding pattern and landing will be delayed.

This often happens to short-haul flights for two reasons. One, longhaul flights obviously get landing priority. Two, the bad weather Instrument Landing System (ILS) means that aircraft have to be spaced wide apart because ILS is prone to interference.

This is because the radio transmitter must have a clear line of sight to the approaching aircraft. Since it’s positioned at the far end of the runway, planes have to land and taxi clear of the runway before a full signal is restored to the next incoming aircraft.

However, a newly developed Microwave Landing System (MLS) will be able to allow up to 30 aircraft an hour to land even in severe fog. The MLS approach technology allows an extra six aircraft an hour to land, meaning that while fog will still cause disruption, its effects will be less prominent.

The existing ILS system was designed over 50 years ago. It relies on two radio signals—one transmitted at the end of the runway and the other at the side. Both are on different frequencies to guide the aircraft down on an approach making a horizontal angle of three degrees with the runway.

The microwave–based system only uses one frequency in a band far removed from that of the ILS system to broadcast the azimuth (distance from runway centre) and angle of descent information to the approaching aircraft.

There’s one interesting and practical point about this new approach system. What the pilot actually sees on the instrument’s displays doesn’t differ from the old ILS technology, so very little training is required to work with the new system.

Okay, so the question of system interference caused by descending aircraft being too tightly queued to each other is solved. Still, air traffic controllers are going to have to make sure that only compatible aircraft are closely in line with each other. A small twin-prop craft sitting close on the tail of a jumbo jet would be severely buffeted by the tremendous amount of turbulence from the larger jet’s engines, making for a very rough approach.

Without wanting to take a Luddite stance on this development, air traffic controllers are also going to have to keep a sharp eye on any landed aircraft that are being slow at exiting the runway for obvious reasons, given the high frequency of landings taking place.

TAGS: Components
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